I don't generally fertilize anything. But I do plant them all in muck. I live next to a river delta, and my native soil is a mix of sand, silt, and river rock. I sift it and use it for planting nearly everything in the water. But below that I use a layer of rich compost from my own piles for the water lilies (usually in fabric pots). For the lotus, I use the silt that I clean out of the sumps every year. It's considerably finer than my soil, and the lotus seem to love it.
Getting them started is still a challenge for me sometimes, though. Of the 5 tubers I got from Bergen this spring, two didn't grow (the most expensive ones, of course). So far the native soil has been most successful for me as a starting media. But when I've got a larger plant to move, as when cleaning out an overgrown lotus, I put them straight in the muck without issue.
I should also mention that there's fish in all of the containers. There are goldfish in these barrels. They last longer for mosquito control than the dunks. Might be some fertilizer coming from them, too.
Oh, and sunlight. More sunlight=more blooms here. And we have some crazy overpowered summer sunlight in the desert here. My large white lotus lives in a 3 foot container sunk into the perennial garden where it misses the first 4 hours of sun each day, and it blooms a bit less there. But it gets lots of debris that breaks down in the water, and its massive flowers put even the roses to shame.